1. Field of the Invention
The present invention relates to a pyroelectric element having a plurality of unit pyroelectric elements obtained by forming a plurality of electrodes on one major surface of a pyroelectric film and forming at least one electrode on the other major surface and, more particularly, to an electrode structure of such an element. The pyroelectric element is suitably used for a pyroelectric infrared sensor.
2. Description of the Prior Art
High reliability is required for intrusion detection pyroelectric infrared sensors used in the fields of securities and automatic doors. Various extensive studies and developments have been made to satisfy the above requirement. U.S. Pat. No. 3,839,640 discloses a pyroelectric infrared sensor wherein a pair of unit pyroelectric elements are connected in parallel or series with each other so as to have opposite polarities, thereby constituting a dual pyroelectric element and improving defects of generating detection errors caused by changes in room temperature and incidence of disturbing external light. Japanese Pat. Laid-Open (Kokai) No. 61-175583 and Japanese Utility Model Laid-Open (Kokai) No. 58-32337 and European Pat. Provisional Publication No. 131,996 disclose electrode structures of dual pyroelectric elements described above, as shown in FIGS. 1A, 2A and 3A, respectively.
The electrode structures of dual pyroelectric elements shown in FIGS. 1A, 2A and 3A will be described below. The same reference numerals denote the same parts throughout FIGS. 1A, 2A and 3A.
Referring to each of FIGS. 1A, 2A and 3A, reference numeral 1 denotes a pyroelectric film; and 2 and 3, light-transmissive or light-absorptive electrodes (to be referred to as light-receiving electrodes hereinafter). The light-receiving electrodes 2 and 3 comprise electrode bodies 2a and 3a and lead portions 2b and 3b extending from the electrode bodies 2a and 3a, respectively. Reference numerals 4 and 5 denote lower electrodes formed on the lower surface of the pyroelectric film 1. The electrodes 4 and 5 comprise electrode bodies 4a and 5a for defining effective light-receiving electrode surfaces and lead portions 4b and 5b extending from the electrode bodies 4a and 5a, respectively. The light-receiving electrodes 2 and 3 are point-symmetrically formed on the upper surface of the pyroelectric film 1. The lower electrodes 4 and 5 are point-symmetrically formed on the lower surface of the pyroelectric film 1. The light-receiving electrodes 2 and 3 and the lower electrodes 4 and 5 are arranged on the upper and lower surfaces of the pyroelectric film 1 to constitute identical electrode patterns.
The light-receiving electrode 2, the lower electrode 4, and a portion of the pyroelectric film which is sandwiched between the electrodes 2 and 4 constitute a first unit pyroelectric element. The light-receiving electrode 3, the lower electrode 5, and a portion of the pyroelectric film 1 which is sandwiched between the electrodes 3 and 5 constitute a second unit pyroelectric element. The first and second unit pyroelectric elements are connected in series or parallel with each other so as to have opposite polarities, thereby constituting a dual pyroelectric element. For example, in order to connect the first and second unit pyroelectric elements in parallel with each other so as to have opposite polarities, a distal end portion 6 of the lead portion 2b is electrically connected to a distal end portion 6 of the lead portion 5b by a conductive adhesive such as a solder, a low melting solder or a conductive paint, or by a U-shaped conductive clip. Input terminals of FETs (Field Effect Transistors-not shown) and lead wires (not shown) are also connected as needed. A distal end portion 6 of the lead portion 3b and a distal end portion 6 of the lead portion 4b are electrically connected to each other in the same manner as in connections of the distal end portions 6 of the lead portions 2 b and 5b.
As described above, although various extensive studies and developments have been made on dual pyroelectric elements obtained by pairing unit pyroelectric elements, pyroelectric infrared sensors having excellent electrical characteristics against disturbing external light cannot always be manufactured by using the conventional techniques of dual pyroelectric elements. In addition, variations in electrical characteristics against disturbing external light are found between the manufacturing lots.